From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Collection of communications and analysis regarding the factors that influence cylinder bore wear, including materials, lubrication, and carburation.
Identifier | ExFiles\Box 132\5\ scan0123 | |
Date | 1st January 1937 guessed | |
--- PAGE 22 --- 22 COMMUNICATIONS ON CYLINDER BORE WEAR sent 1939 models, particularly in view of the fact that one maker had adopted steel liners pressed from the sheet. Particulars given on p. 2 for car A enabled him to identify the engine. Recently he visited the manufacturer's English factory and was there shown a number of cylinder blocks from engines which had run around 60,000 miles. The average wear in these blocks was 0·006-0·007 inch, or about 10,000 miles per 0·001 inch. He was assured that such wear rates were their usual average. Further, he was advised that so good was the reputation of this car for cylinder bore wear that should an engine require reboring after 40,000 miles, as a result of misuse, the firm's customers would not expect to bear the whole cost of such work. Such facts were particularly interesting as the lower cylinder wear of American cars had been attributed to the fact that in America greater distances were travelled between stops. On p. 4, the author mentioned that a uniform cylinder wall temperature was desirable. In his own experience it was of paramount importance, particularly in air-cooled engines, which appeared, as a class, to be particularly prone to excessive cylinder wear. He recently encountered such a case, where a side-valve air-cooled engine could be driven with practically no cylinder wear provided that speed did not exceed 35 m.p.h. Higher speeds, and in consequence greater heat, gave cylinder wear of 1,000 miles per 0·001 inch, the whole wear being located upon the non-thrust side of the bore, i.e. upon the high-spot region, due to the shielding effect of the valve chest. Individually cast piston rings of the cast-to-form high-tension type were installed, together with an arrangement giving very copious cylinder lubrication. This had the effect of improving the wear from 1,000 miles per 0·001 inch to 5,000 miles per 0·001 inch, together with an improvement in lubricating oil consumption of 500 miles to the gallon. He felt that this improvement was largely due to the increased oil cooling of the hot-spot area; he would be glad of the author's comments upon this point. On p. 5 the author drew attention to the increased wear obtained when low-grade alcohol fuels were used. He himself had observed in a carefully controlled "one-make one-service" fleet an average cylinder wear of 3,700 miles per 0·001 inch being changed to 5,300 miles per 0·001 inch by changing from cheap alcohol fuel to a straight No. 1 blend. The service referred to entailed much cold starting and idling. On p. 10 the author mentioned that in order to combat wear by sheer hardness, it was necessary to use the equivalent of nitrided steels. In this country, the whole range of hardnesses, with Brinell numbers ranging from 220 to 900 (the latter being for nitrided cast iron liners), had been tried. At present engine builders were obtaining better wear from an all-pearlitic iron with a Brinell hardness of about 250 than they did from other and harder materials. In his own experience sheer hardness did not appear to be the criterion for good wear. He agreed entirely that blow-by was one of the most usual causes of clogged oil rings. In the pre-war period of good sealing compression rings, he thought that an adequate oil supply through the oil scraper rings from the beginning was one of the best ways of keeping these rings clear and cool, in order to avoid deposition of impurities and subsequent reduction of scraping efficiency. He also believed that if satisfactory oil mileage could only be obtained by the use of one scraper ring having excessively narrow scraping lands, an improved oil consumption in respect of mileage would result from the use of two wide-land oil scraping rings, by virtue of the lower rates of wear on the wide lands. Oil scraper rings in the skirt of the piston should be used with caution, as there was always the risk of cutting off the oil circulation at a point too low down the piston. The gudgeon pin relief upon pistons could act as an effective and undesirable sludge trap if this condition obtained. In his paragraph on piston ring practice (p. 14), the author mentioned the use of special piston ring materials. In his own opinion there was a tendency to use special materials before thoroughly exploring the possibilities of a normal material. He had in mind a high-output high-speed supercharged engine, which was operating with great success, using rings of standard cast iron composition, all with a pressure distribution suited to the particular conditions. In no case should special materials be adopted until it could be confidently felt, by actual development work, that the optimum gas sealing conditions had been obtained. Bad blow-by naturally affected ring wear. In his own experience, working upon the lines indicated by the author definitely improved piston wear. More intensive development work in connexion with piston rings and bore lubrication was needed than had been carried out in the past. Such work would be well repaid by its results in service. Mr. H.{Arthur M. Hanbury - Head Complaints} J.{Mr Johnson W.M.} YOUNG (Sheepbridge Stokes Centrifugal Castings Company, Ltd.) wrote that it was invigorating to read the thoughts of someone not only able to leave the beaten track but having the courage to disagree with it. The plain language employed by the author enabled his paper to be digested even by those who were not engineers. He himself had always been in agreement with the author's remarks con- --- PAGE 3 --- CYLINDER BORE WEAR 3 *Factors Affecting Bore Wear.* The factors affecting bore wear are complex, and, although an engine may include all the known means of protection against wear, wear still occurs, even under conditions considered to be favourable. Such engines may be fully warmed and, with a minimum of starts, may be operating on a fast or mixed schedule. It is this fact which prompts the author to argue against over-emphasis on corrosion. Curve A of Fig. 1 indicates cylinder wear rate for a most interesting case. This engine was run for 20,000 miles under favourable conditions. The wear varied in the cylinders, the maximum being at the front, the minimum at the rear. The amount of oil supplied to each bore was as nearly the same as possible. Further, the amount of oil on the bores was far in excess of that in any other pressure-fed engine, being designed to equal the amount of oil thrown by splash lubrication. The engine incorporated a water thermostat with a terminal temperature of 160 deg. F.{Mr Friese}, the blow-by was reduced to ¼ cu. ft. up to 4,000 r.p.m., and the crankcase ventilator maintained a vacuum equal to ¼ inch water gauge in the oil sump and crankcase, so that contaminating gases were reduced to a minimum. What are the critical variables, and particularly, what are the variables that can exist in one engine? *The Effect of Carburation.* A year ago, Mr. W. L. Fisher told the author that carburation was just as important a factor in bore wear as any other. Mr. C. G.{Mr Griffiths - Chief Accountant / Mr Gnapp} Williams, in discussing the author's last paper on this subject, stated he had suspicions that the trans-atlantic manifold heaters might have something to do with the difference between American and English bore results. The author innocently replied that he did not believe English carburation was so bad. Those designers saw further than he did, because to-day, he has to face the fact that a difference between a mixture of 12/1 ratio and one of 14/1 ratio, although in the part-throttle range, may affect bore wear at the startling rate of 3 to 7 times. The words "may affect" are used advisedly, as the results, though obtained with care, are from one type of engine, and other variables may mitigate this effect; for example, the engine in question was under-oiled. Lean mixtures favour low wear. This is upsetting, because the oxidizing atmosphere would be expected to affect bores adversely. However, despite this, lean mixtures show up, not merely favourably, but to an outstanding extent. In what way does a weaker, and therefore, drier mixture help, and incidentally why by as much as 7 to 1 on an under-oiled engine? It is believed that the oil film is weakened by dilution from a rich mixture. This oil film, being too thin, is more sensitive to attack, and thus a difference in mixture richness of 16 per cent may have a major effect. The above tests were also carried out with a minimum of adverse conditions. The car was kept in a heated garage and the mileage was run with long intervals between starts. Fig. 2 shows the comparative mixture ratios of the two 30 mm. "Zenith" carburettors involved in this test. One represents 1936 British carburettor calibration, the other a rearrangement of the calibration to suit the engine demand as determined at Vaux- Fig. 1. Rate of Wear and Miles per Gallon of Oil A Rate of wear for each cylinder. B Miles per gallon of oil. C Miles per gallon of oil. Fig. 2. Comparative Mixture Ratios of two 30 mm. Carburettors 14 h.p. Model "D" carburettor _______ Full throttle – – – – – Part throttle | ||